In ac systems, locally isolated power supplies are used to provide dc voltage and current for various purposes. One application of this type of power supply is to provide gate firing energy to individual thyristors in a series connected thyristor array. This type of thyristor array is commonly used in static VAR generators to switch capacitors or inductors onto the ac system. For example, where the thyristors are used as a capacitor switch, the currents in the switch may occasionally reach 20 to 40 times the current rating of the thyristor. At times the current may be as low as one-tenth the rating of the thyristor. It is convenient to express these excursions in the current range on a per unit (P.U.) basis where one per unit is equal to the current rating of the thyristor or other device which is employed. For example, the dynamic range of the ac current source can extend from 0.1 P.U. to 40 P.U. To operate in the capacitor switch, the power supply for the gating circuit must therefore be designed to provide a required constant output when the input to the power supply can vary over a range of 400 to 1. Typically, the power supply is connected to the ac system using a current transformer. Because the load represented by the firing circuit of the thyristor switch is relatively constant while the current in the ac system is variable, the transformer is designed to operate at the minimum current level expected. A surplus capability then exists at any currents higher than the design level and this must be bypassed if the output voltage is to remain constant. For example, where a load such as the gate firing circuit is to be supplied with 24 volts at 250 mA and the thyristor has a rated average input current of 2000 amperes which is equal to 1 P.U., then the minimum design current, assuming a 0.1 to 40 P.U. dynamic range for the ac system, would be 200 amperes. For the current transformer a turns ratio between the secondary and primary windings of 800:1 is required. At the rated average input of 2000 amperes, this current transformer will be furnishing ten times the required current (i.e., 2.5 amperes). If a zener diode is used to regulate the output voltage of the power supply, it will dissipate 54 watts continuously. This figure is determined by multiplying the output voltage of the power supply by the difference between the actual current of 2.5 amperes being furnished and the desired 250 milliamp output. Under surge current conditions of 20 P.U. (40,000 amperes), the dissipation will transiently approach 1200 watts. To handle this sort of dissipation reliably, special zener diodes are required and careful attention must be given to thermal operating conditions. A power supply having reduced current and heat dissipation during surge current condition would therefore be advantageous. One object of the present invention is to provide a power supply operable from an ac current source having a large dynamic range in which the excess current or power dissipated therein is reduced.